The present invention relates generally to the field of navigation and, in particular, to identification of navigational markers.
Radar is commonly employed in many navigational systems, particularly in the areas of aeronautical and marine transportation. In marine transportation, radar is used for navigation and collision avoidance. Radar is a necessary component of marine navigation, as it allows the operator of a vessel to follow a safe course along a body of water, and to adjust the course of the vessel in avoidance of objects and obstructions. In marine navigation, radar is used in telemetry, to sense and determine range (distance) from, and/or bearing (direction) of, natural and man-made objects, the positions of which are known. These objects include natural geodetic features, such as shorelines, rocks, islands, and man-made objects such as bridges, jetties, breakwaters, harbors, and others. Radar is also essential in collision avoidance by locating and identifying unknown objects, such as other vessels, ice masses, debris, logs, and others. Warning markers and buoys, placed to divert marine traffic from hazards, such as rocks, sandbars, submerged wrecks, and the like, are also detected by radar. Buoys and markers are commonly placed to ensure safe and efficient navigation by guiding marine traffic along designated paths in bodies of water, such as the main channel of a river.
In most marine navigation systems, radar is pulse modulated, which creates a pulse emission sequence. A radar transmitter, which is coupled to an antenna, generates a very short pulse of radio-frequency (RF) energy. The duration of the pulse is typically on the order of 1 milli-second. The RF energy radiates outward from the antenna, wherein the radiated RF energy is focused into a relatively narrow directional beam. When the RF energy strikes an object, a portion of the RF energy is reflected (echoed) by the object and the antenna receives the reflected RF energy. A radar display screen synthesizes the RF energy reflections into visual representations of the object reflecting the RF energy. The display screen shows the approximate range and bearing of the object in relation to the radar antenna. The radar transmitter generates another pulse, and the sequence is repeated.
There are several factors that affect the ability of radar systems to effectively sense objects. The radiated RF energy may be diffracted by other objects in the area, creating abnormalities in the display image. The radiated RF energy may also be subject to unwanted reflections by unforeseen objects in its path, such as waves and turbulence on the surface of the water, creating xe2x80x9cfalsexe2x80x9d images. The amount of unwanted reflections typically increases at higher radio frequencies. Another condition that affects radar is atmospheric attenuation or absorption of the radiated RF energy, which reduces the echo intensity. A further limitation of radar is that the radiated RF energy tends to travel primarily in a straight line, thus limiting the effective range of radar to approximately the line of sight from the antenna outward to the horizon. The effective range of radar thus becomes a function of the height of the antenna above the surface of the water. It should be noted that the radiated RF energy has some ability to refract, or bend, in the atmosphere, thus increasing the effective range of the radar beyond the line of sight. However, the atmospheric refractivity of radar greatly diminishes at higher radio frequencies.
The resolution of radar is a function of frequency. Radars operating at higher radio frequencies have shorter wavelengths and are capable of detecting smaller objects, however, higher frequency radars have shorter ranges and are more susceptible to unwanted reflections.
Buoys are floating aids to navigation. Buoys mark channels and harbors, indicate shoals and obstructions, and provide warnings of dangers such as rocks or shallow water. Buoys are typically moored via cables or chains to the bottom of a body of water. One type of buoy contains a lamp to increase visibility. The light emitted by the lamp may be a certain color, depending on its intended use. For example, green and red lamps are used respectively to mark the left (port) and right (starboard) sides of a channel as viewed in the upstream direction. Other colors are used to mark danger buoys. Another type of buoy employs an audible device, such as a bell, gong, or whistle, to alert operators of vessels. Yet another type of buoy uses a radio responder, called a racon. The racon emits a radio signal when triggered by radar from a vessel. A radar receiver on the vessel detects the radio signal emitted from the racon. The radio signal emitted from the racon is displayed on the radar display screen. However, the information provided from the racon to the radar system only provides an approximate range and/or bearing of the buoy. The information does not identify the type of buoy or the purpose of the buoy.
A very common type of buoy is a simple floating marker, unlighted, and without an audible device or racon. The standard unlighted buoy is detected either visually, or by reflecting radar. The standard unlighted buoy bears markings to denote its specific purpose. For example, a buoy bearing a green marking may be used to direct traffic to the right of the buoy, thus the buoy is seen on the left, or xe2x80x9cportxe2x80x9d side of the passing vessel. Certain unlighted buoys are differentiated by shape. Shapes of buoys include conical, cylindrical, spherical, and others.
Some buoys are equipped with a radar-reflective device to enhance their detection by radar systems. The radar-reflective device is typically an object constructed of light-gauge sheet metal, formed in such a manner as to reflect a portion of the radar signal that strikes it. Radar-reflective devices are typically affixed to the outside of a buoy, and are often subject to corrosion and damage. Nevertheless, buoys of this type, lacking a racon device, tend to be poor reflectors of radar. When radar detects a buoy or marker, the reflected image is sometimes displayed as an unidentifiable shape on the radar screen. The radar system cannot distinguish the color or markings of the buoy, requiring the vessel operator to visually identify the buoy. In conditions of poor visibility, visual identification of the buoy may occur too late for safe navigation. Due to the limited resolution of the radar, the image on the radar screen may be indistinguishable from the image of another object, such as a vessel or floating log. Speculation as to the identity of the reflected image creates an undue hazard in navigation, as the vessel operator is unsure as to which course to take in relation to the unidentified object.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in identifying navigational markers in transportation systems.
The above-mentioned problems with identifying markers in radar navigation and other problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification.
In one embodiment, a navigational marker includes a body and a polarized radar-reflective material affixed to the body. The polarized radar-reflective material identifies the navigational marker.
In another embodiment, a radar receiving apparatus includes a receiving antenna, a circuit connected to the receiving antenna for detecting a radar signal and identifying a distinctly polarized radar reflection from a navigational marker, the circuit generating information identifying the navigational marker, and a display connected to the circuit to output the generated information.
In yet another embodiment, a method includes transmitting a signal to a navigational marker, polarizing a reflected signal from the navigational marker, receiving the polarized reflected signal at a radar receiver, and identifying the navigational marker by decoding the polarized reflected signal.
Other embodiments are described and claimed.